56 THE CIRCULATING LIQUIDS OF THE BODY 



Many other methods have been devised on the principle of in- 

 jecting a known quantity of some substance into the circulating 

 blood, and then, after an interval has been allowed for mixture, 

 determining the change produced in a sample. Thus, the specific 

 gravity of a drop of blood having been measured, a certain quantity 

 of a solution of sodium chloride isotonic with the plasma may be 

 injected into a vein, and the specific gravity again determined. Or 

 the electrical resistance of a small sample of blood may be measured 

 before and after injection of a given quantity of isotonic salt solution. 



The quantity of blood in the body was greatly overestimated by 

 the ancient physicians. Avicenna put it at 25 lb., and many loose 

 statements are on record of as much as 20 lb. being lost by a patient 

 without causing death. By Welcker's method the proportion of 

 blood to body- weight has been found to be in the dog i : 13, cat i : 14, 

 horse i : 15, frog i : 17, rabbit i : 19, fowl i : 20. In new-born 

 children the proportion was i : 19, in adult human beings (executed 

 criminals) i : 13. The total mass of the blood in a living man has 

 been estimated by causing the person to inhale a known volume of 

 carbon monoxide mixed with oxygen or air, and then determining 

 in a sample of blood taken from the finger the percentage amount to 

 which the haemoglobin has become saturated with carbon monoxide. 

 All that remains is to estimate the volume of carbon monoxide (or, 

 what is precisely the same thing, the volume of oxygen) which 

 100 c.c. of blood will take up. This latter quantity is called the 

 percentage oxygen capacity. From these data the total volume of 

 the blood can be calculated. If the volume is multiplied by the 

 specific gravity the mass is obtained. 



Thus, if the haemoglobin was found to be 25 per cent, saturated with 

 carbon monoxide after the person had absorbed 150 c.c. of that gas, 

 the whole of the blood would require 600 c.c. of carbon monoxide to 

 saturate it completely. If the percentage oxygen capacity was 20, 

 20 c.c. of oxygen or carbon monoxide would be needed to saturate 

 100 c.c. of blood. Therefore the total volume of the blood would be 



IOO 



600 x =3,000 c.c. And the mass, if the specific gravity was 1-055, 



would be 3,000x1-055 = 3,165 grammes. According to this method 

 the blood on the average in man constitutes only 4*9 per cent., or 

 jjj.., of the body-weight (say, 3|- kilogrammes in a 70 kilo man), varying 

 in fourteen persons between ^ and y 1 ,;. There is reason, however, for 

 thinking that the method at least, as hitherto employed underesti- 

 mates the quantity of blood. According to Dreyer, the blood volume 

 is a function of the surface of the body, so that the smaller and lighter 

 animals in any given species have a relatively greater blood volume 

 than the larger and heavier individuals. Accordingly, he considers 

 that the practice of expressing the volume of blood as a percentage of 

 the body-weight should be abandoned. 



Fig. 16 (p. 54) illustrates the distribution of the blood in the 

 various organs of a rabbit. The liver and skeletal muscles each con- 

 tain rather more than one-fourth; the heart, lungs, and great vessels 



